Several studies presented Wednesday at the American Heart Association annual meeting, in Orlando, Fla., addressed different aspects of this promise.

One study found that patients' own stem cells, when injected into the heart after treatment for a heart attack, improved the heart's ability to pump effectively. But this finding is fairly preliminary.

"We're still learning. The area of stem cells still needs lots of understanding," said Dr. Robert Bonow, immediate past president of the heart association and chief of cardiology at Northwestern Memorial Hospital in Chicago. "What are the right kinds of cells? How do we train them into cells that are viable and make people better? What are the right kinds of patients? We really don't know that. We're waiting for more data."

The first study involved 80 patients who had recently received treatment (clot-busting drugs and angioplasty) for an ST elevation heart attack (STEMI), the more serious type of heart attack. The patients were randomly assigned to receive either injections of their own bone marrow cells or a placebo.

After six months, patients who had received bone marrow cell therapy saw their global ejection fraction, a measure of the heart's pumping function, improve from 59 percent to 67 percent. The ejection fraction remained unchanged in the placebo group.

"In conclusion, intracoronary injection of autologous [from the patient] bone marrow cells improves left ventricular systolic function in STEMI patients who are treated initially with thrombolytic therapy followed by [angioplasty] two to six days after a heart attack," said Dr. Heikki Huikuri, lead investigator of the trial and professor of medicine and director of the cardiology sector at the University of Oulu Hospital in Finland. "This appears to be a safe therapy."

Bonow added: "The findings are interesting but preliminary. The ejection fraction showed a meaningful increase, but they were normal to begin with. We need more time and more information."

The study was funded by the Finnish Academy of Science and Boston Scientific Inc.

Two other studies presented Wednesday in late-breaking sessions at the AHA meeting showed less convincing results. These studies looked at a different population than the Finnish study: patients with chronic scar tissue from a prior heart attack.

The first study involved injecting patients' own skeletal muscle stem cells directly into the heart. The trial, in 23 heart-failure patients, was primarily designed to assess safety, but the researchers also looked at effectiveness.

"These cells are not designed to be heart tissue, but they do contract and have some electrical activity," Bonow explained.

At one year, the procedure proved safe and, the study authors said, resulted in improvements in pumping function and better quality of life.

The study, however, was not "blinded," meaning that any improvements could be due to a placebo effect. Decreases in the heart's size were minimal, Bonow said.

"This doesn't lead anywhere right now," Bonow said.

The study was funded by Mytogen Inc., a biotech company developing cell-based therapies.

The final study, which involved injecting patients' own stem cells into heart scar tissue, found that it did not affect heart muscle functioning.

"It was safe but didn't improve systolic function, didn't reduce infarct size and didn't influence global left ventricular function," said study lead author Dr. Manuel Galinanes, professor of cardiac surgery at the University of Leicester in England.